1. Field of the Invention
The invention relates to a screw element with an outer radius Ra and a core radius Ri for multiscrew extruders with co-rotating and intermeshing screw shafts, in particular for twin-screw extruders, which has in axial cross section through its longitudinal axis a profile which has at each of the two axial end faces only a screw flight corresponding to a conventional single-start screw element for intermeshing screw shafts, the surface of which flight, between a left and a right flight edge, is part of a surface of a cylinder with the outer radius Ra, each end face having a circular root with the core radius Ri of the screw shaft and a left and a right flank, which joins the root to the left and right flight edge, respectively.
2. Description of the Related Art
The design principles for creating screw elements for co-rotating and closely meshing multiscrew extruders, which are also referred to as Erdmenger profiles, have been known for many years. A corresponding description can be found for example in the book “Der Doppelschneckenextruder, Grundlagen und Anwendungsgebiete” [The twin-screw extruder, principles and areas of application], published by VDI Verlag GmbH, Dusseldorf, 1995 (pages 10-30). In the illustration 1.4 (page 14) of this publication there is shown, for example, an axial cross section of the profile of a single-start screw element of the type mentioned at the beginning.
For the dispersive and distributive mixing of additives, for example, or other components into plastic compositions, usually kneading blocks which comprise a plurality of kneading disks with an Erdmenger profile, arranged axially one behind the other and offset angularly with respect to one another, are used. The kneading disks are respectively arranged in pairs, lying opposite one another on the two screw shafts of the respective twin-screw extruder, and closely intermesh. The mixing process in conventional kneading blocks is to be regarded as a random process, i.e. the mixing work performed in individual volume elements varies in intensity. Therefore, to achieve a high degree of homogeneity of the mixture, considerable mechanical energy has to be expended to ensure that, as far as possible, every volume unit also undergoes shearing. On the basis of an individual kneading disk, a relatively small proportion of the material to be handled is in each case sheared extremely intensely, while by far the greatest part of the material evades the shearing gap between the shearing disk and the barrel wall and is consequently sheared only little. For this reason, to ensure a high degree of homogeneity of the mixture, either very long kneading blocks of the known type or else high rotational speeds are required. In any event, considerable mechanical energy is expended and is introduced in the form of heat into the material to be handled. In particular during the processing of rubber mixes, the generation of relatively large amounts of heat is extremely undesirable.
DE 42 39 220 A1 discloses a twin-screw extruder with two identical, closely meshing and co-rotationally driven screw shafts, which are arranged in the bores of a shared barrel. The screw shafts are provided with kneading disks, which have a three-start shaft cross section, that is to say have three flight lands. The distance of the flight lands from the inner surface of the barrel bore and the width of the flight lands vary. The flight land with the greatest flight land width has in this case the smallest distance from the inner surface of the barrel bore. The screw elements known from this document are of a three-start form over their entire axial length.